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Investigation into the Combustion of Alcohols.

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Investigation into the Combustion of Alcohols Aim: To investigate the amount of energy given off by the combustion of Alcohols. Planning: This investigation involves burning alcohol in the air. According to dictionary.com an alcohol is "Any of a series of hydroxyl compounds, the simplest of which are derived from saturated hydrocarbons, have the general formula CnH2n+1OH, and include ethanol and methanol." The alcohol reacts with the oxygen in the air to form the products water and carbon dioxide. This reaction is exothermic, as heat is given out. This is because the reactant energy is more than the product energy. The formulaic representation of this difference is ?H. The energy is given out when forming the bonds between the new water and carbon dioxide molecules. The heat content, also known as enthalpy, is negative in exothermic reactions because the energy (?H) is 'lost' as heat. Enthalpy is defined as "a thermodynamic function of a system, equivalent to the sum of the internal energy of the system plus the product of its volume multiplied by the pressure exerted on it by its surroundings." For any reaction carried out directly at a constant pressure, the heat flow is exactly equal to the difference between enthalpy of products and that of the reactants. This can also stated as Qp = Hp - Hr = ?H In this, Qp is the heat flow at constant pressure, Hp is heat energy of products, and Hr is the heat energy of the reactants. ...read more.


The height of the beaker from the wick 6. Same set of scales 7. Weigh the spirit burner with the lid on. The variable that must be changed is: The type of alcohol used Results Initial temp (C) Final temp (C) Temp rise (C) Methanol 17 47 30 Ethanol 18 48 30 Propanol 19 49 30 Butanol 21 51 30 Hexanol 17 47 30 Initial burner (g) Finish burner (g) Mass burnt (g) Methanol 214.56 212.21 2.35 Ethanol 133.9 131.7 2.2 Propanol 330.93 328.9 2.03 Butanol 184.8 182.61 2.19 Hexanol 287.54 285.83 1.71 Analysis In order to calculate the amount of energy evolved, I will use the formula Energy evolved = Mass x Rise in temperature x SHC Energy evolved = 250g x 30�C x 4.2 Energy evolved = 31500 As you can see all of the alcohols will have the same amount energy evolved because all the numbers that are filled in the formula remain constant for each alcohol and the same numbers are applied for each individual alcohol. Below is a table showing the amount of energy evolved in each case... To find out how much energy is produced per gram we use the formula... Energy per gram of fuel = Energy evolved x Mass of fuel burnt Energy per gram of fuel = 31.5kj x ? Below is a table showing how much energy is produced per gram when burning the alcohols in question... Type of alcohol Energy per gram (kJ) ...read more.


Since heat is given out when bonds form, less energy is therefore given out by incomplete combustion. To overcome this problem, I would have to make sure a sufficient supply of oxygen was involved in the reaction. Evaporation of water may also have had an effect, therefore there would be less water to heat, making the water hotter, but also some of the energy would be lost during the evaporation process. Also, depending on the alcohol, the flame size changed and therefore it was a different distance away from the beaker each time. Given that only 5 alcohols were tested, and the inaccuracies of the experiment, I would say that the evidence is not strong enough to draw firm conclusions from, however, as stated earlier, the consistency of the results seems to imply that roughly the same amount of heat was lost each time. Therefore, this experiment is valid when considering the proportionality of the ratio of molecular size to energy per mole, however, it cannot be used to accurately indicate the amount of energy given off. If this experiment was to be done again, then all the possible sources of error mentioned would have to be counteracted and controlled, as well as using a much wider range of readings of many more alcohols, burn them for different periods of time, heat different substances other that water, investigate the other variables. I would also take many more readings so that a more accurate average could be taken. Next time reducing heat lost would be my main priority. Improving insulation techniques would be a valuable asset in obtaining the most reliable data I could. ...read more.

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